AVIATION AIRBOUND N
Ultrasonics allows nondestructive aviation inspection by seeing the unseeable. By Mark Robins
ondestructive ultrasonic testing uses beams of high- frequency sound waves to detect subsurface flaws, measure thickness and evaluate material properties. When used to
inspect aerospace components, ultrasonic testing is an essential facet of any maintenance and manufacturing program influencing safety, quality assurance and cost. “In those facilities which manufacture aerospace parts, ultrasonic NDT is a quick and reliable in-process method to determine whether parts are correctly fabricated with respect to porosity levels, disbonds, and delaminations,” says Bob Lasser, CEO at Imperium, Beltsville, MD. “With respect to maintenance facilities for in-service aircraft, ultrasonic NDT is an important tool for detecting subsurface foreign object damage (FOD), barely visible impact damage (BVID), and cracks near fasteners. For in-service inspections, ultrasonic instruments can detect flaws without time- consuming and costly disassembly of the aircraft.” Aircraft parts are designed to be as light as possible while still performing their intended function. This means components carry very high loads relative to their material strength, but even small flaws can cause a safety-critical component to fail. Ultrasonics can detect these very small defects which are too small to be detected by visual means.
The most common ultrasonic-inspected aviation components
are fuselage panels, wing panels, helicopter rotor blades, turbine fan blades and radomes. Also, “potential airframe, landing gear and turbine engine component problems can be detected by ultrasonic inspection,” says Tom Nelligan, senior applications engineer at Olympus NDT in Waltham, Mass. “This allows for the necessary corrective action by the manufacturers, airlines, MROs and local aviation maintenance facilities. By utilizing ultrasonic instrumentation along with specific transducers and sensors, high-frequency sound waves can be used by a trained and certified operator to locate cracking, delamination and thickness changes due to skin repair patch processes, corrosion or blending operations in metal and composite parts.” Other defects capable of being found are shrinkage, stringers, hot tears, air pockets, lack of fusion, lack of penetration, incomplete penetration, incomplete fusion, crushed cores and inclusions.
Understanding ultrasonics
A typical ultrasonic inspection system uses high-frequency sound energy to conduct evaluations and take measurements. It is made up of several components, including a pulser and receiver, transducer and display devices. The pulser and receiver produces
Above: The new RotoArray is a manually operated, phased array, ultrasonic inspection system allowing rapid scanning of composite surfaces and structures. (Photo courtesy of GE Measurement & Control)
Below: This operator is checking for structural damage on landing gear with a Phasor XS portable flaw detector. (Photo courtesy of GE Measurement & Control)
a high-voltage electrical pulse while the transducer generates high-frequency ultrasonic energy. In order to create this energy, a transducer contains a thin disk made of a crystalline material with piezoelectric properties, such as quartz. The frequencies commonly used in ultrasonic testing are ten to one hundred times higher than the highest frequency or pitch than can be used by the human ear (0.2-20 MHz versus 20 KHz).
At those frequencies sound waves become highly directional,
and reflection and refraction occur when they interact with interfaces of different acoustic properties. The vibrational energy picks up discontinuities such as a crack in the wave path, reflecting back some of the energy from the flawed surface. The transducer transforms the wave signal to an electrical signal and displays it on a screen.
The ultrasonic devices, instruments, gauges and sensors utilized in the aviation NDT industry cover a wide range. “Today one uses a very simple hand-held gauge in Go / No-Go 35RDC (ramp damage checker), hand-held precision ultrasonic thickness gauges, portable ‘conventional’ ultrasonic flaw detectors, inspection yokes, small- and large-ultrasonic immersion tank systems, large gantry ultrasonic squirter systems and many different designs of ultrasonic transducers, sensors and roller probes,” says Wayne Weisner, business development manager of aerospace at Olympus NDT. Ultrasonic instrumentation is portable and may be operated under any weather conditions. The primary consumable for most UT processes is the couplant which may be water or oil depending on the application. However, a laser ultrasonic inspection system requires no coupling medium. With it, generation and detection of the ultrasound can be made at a distance with laser light, without any physical contact with the surface of the aircraft component being inspected. The ultrasonic probing pulse is always emitted in a direction normal to the component’s surface, regardless of the laser beam’s angle of incidence. There is no need for normal incidence to the surface of the sensor, as is required for conventional pulse-echo UT systems. Hence, no particular knowledge of the component’s shape is needed prior to the inspection. There are manual and automated UT testers. With manual testing, “results evaluation is to the discretion of the trained operator with usually no trackable result,” says Albrecht Maurer, Alzenau Site Leader at GE Sensing & Inspection Technologies in Alzenau, Germany. “With automated testing, equipment can be customized to best meet the testing criteria. For large components, this is the most economical solution. In addition, automated testers’ output is a cartography of the ultrasonic interaction, which can be stored in the parts records.” In recent years, the aviation, aerospace manufacturing and in-service industry have encompassed more of the “advanced” ultrasonic phased-array (UTPA) instrumentation and multi-element array sensors. Weisner believes the implementation and use of this
Aviation Maintenance |
avm-mag.com | February / March 2013 33
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